1. Field of the Invention
The present invention relates to a method of preventing infectious diseases, a method of enhancing an infectious disease-preventive action of cystine or a derivative thereof, a composition, food and feed, which are effective for the prevention of infectious diseases.
2. Discussion of the Background
Respiratory infectious viruses such as influenza virus, rhino virus, corona virus, parainfluenza virus, RS virus, adeno virus, reo virus and the like are causal viruses that grow in epithelial cells of from nasal cavity to pharynx, trachea, bronchus and to lung, and produce cold syndromes, namely, diseases often seen in daily lives with manifestations of cold symptoms. Most develop and recover from mild symptoms of rhinitis, pharyngitis and the like, but some develop severe complications such as pneumonia and the like, sometimes with fatal outcome, and these diseases require caution. When immunity decreases due to stress and the like and the living organism is not able to resist infection, a symptom appears. Currently, the treatment methods are generally symptomatic and non-prescription drugs such as cold remedy and the like, which contain antipyretic analgesics (e.g., acetaminophen and the like), drugs that suppress allergic symptoms (e.g., chlorpheniramine maleate and the like), antitussive and the like (e.g., methylephedrine hydrochloride and the like), are mostly taken. As the current situation stands, when the symptoms start to get rather worse, people go to hospital and receive the above-mentioned cold remedy, antimicrobial agents dealing with concurrent development of bacterial secondary infection and vitamins from doctors.
As a preventive prescription, preventive vaccination of inactivated vaccine against influenza prior to epidemic, which is the most important of these causal viruses, is given. There are many strains of influenza virus based on different surface antigens, which are susceptible to antigenic variation. Thus, inconsistency in the antigen structure between vaccine strain and epidemic strain is a significant problem. When a strain different from the vaccine inoculated based on prediction is in epidemic, the vaccination becomes ineffective. In addition, respiratory infectious viruses are known to include many kinds as described above, and each virus has many subspecies. Thus, a virus type selective preventive method may or may not be effective and, in some cases, the epidemic virus type (subspecies) may prove wrong.
Moreover, as a preventive method of infectious disease with respiratory infectious viruses, a gargle liquid of an iodine preparation has been used, but it is defective in that it has a distinctive bitter taste. While amantadine and rimantadine having an anti-influenza virus type A action have been used in the US for the prevention of infection (MMWR, 44, RR-3, 1995), they are associated with many restrictions on general prevalence from the aspects of the risk of emergence of resistant strain and possibility of side effects. Under the circumstances, preventive methods of infectious diseases with respiratory infectious viruses are not necessarily sufficient and there are problems yet to be solved.
On the other hand, there are several reports on the preventive and therapeutic agent for virus infectious diseases using sulfoamino acids such as cysteine and the like or a substance containing the same, such as a composition for prevention•treatment of influenza virus infection using a compound selected from glutathione, glutathione disulfide, ascorbate-2-phosphate and N-acetylcysteine (WO98/30228), a preventive agent of virus infectious diseases, which contains cysteine as an active ingredient (JP-A-2001-213774, WO01/56561), utilization of cystine, oxidized glutathione, S-alkylcysteine sulfoxide for the treatment of patients with human immunodeficient virus (EP0764442A1) and the like. However, it is undeniable that a more efficacious preventive agent can make a greater contribution to the medical field.
For the treatment•prevention of infectious diseases, administration of chemotherapeutic agents is general. However, new problems have been produced, such as emergence of bacteria that acquired resistance to pharmaceutical agents. In addition, the incidents of infection as a result of lower resistance to infection due to the use of immunosuppressants in more frequent occasions of used in recent years for the treatment of diseases have been increasing. Thus, appearance of an effective preventive agent has been desired.
Theanine is a γ ethylamide of glutamic acid and an amino acid contained at high levels in green tea. As regards its physiological action, there are many reports relating to the central action such as relaxing action, α wave induction action, suppression of caffeine rush and the like. As for antibacterial action of theanine, reports have been documented on the suppression of trichomonas growth by the addition of extremely high concentration of theanine in a vitro cultivation system (Nature, Vol. 166, 352, 1950), suppression of growth of Staphylococcus aureus (J. Biol. Chem. Vol. 171, 387, 1947), suppression of growth of Staphylococcus aureus and Escherichia coli (Chem. Pharm. Bull. Vol. 28, 3549-3554, 1980), and the like. On the other hand, a report has concluded that, what is mainly conducive to the antibacterial activity against B. subtilis, which is enhanced by heating the green tea extract, is not theanine but catechins (Journal of the Japanese Society for Food Science and Technology, Vol. 47, No. 9, 708-715, 2000). Moreover, there is no report on the effectiveness of theanine in the prevention of infectious diseases.